Internal combustion engines used on vehicles produce a mixture of different gas species in their exhaust streams. It may be useful to characterize the concentration of a particular gas species in the gas mixture. For example, an oxygen sensor may be used to determine air/fuel ratio and to provide a feedback signal for controlling the engine to achieve a desired air/fuel ratio. A gas species sensor may also be useful for controlling an exhaust aftertreatment device. For example, an SCR catalyst may utilize urea provided by a urea delivery system to reduce NO and NO2 in a diesel engine exhaust stream, and an ammonia (NH3) sensor may be used downstream of the SCR catalyst to control the rate of urea delivery. Knowledge of a gas species concentration may also be useful in providing required On-Board Diagnostic (OBD) information.
A gas species sensor produces an output that varies with the concentration of the gas species. However, the sensor may be sensitive to a number of different gas species. As a result, the output of the sensor may be influenced not only by the gas species that it is desirable to measure, but also by other gases in the mixture.
It is therefore desirable to provide a method for determining the concentration of a given gas species in the presence of other gas species.